Plant-Animal Interactions

Paola Ugoletti (PhD student)

Research Profile

This thesis aims to improve knowledge of invasion processes through investigation of the invasive strategies of congeneric Impatiens species that vary in invasive status in Ireland. Since a range of physiological traits promote plant fitness, many studies have attempted to compare these traits among invasive and non-invasive species. Meta-analysis and desk-based reviews offer the advantage of highlighting which traits are consistently associated with plant invasiveness. However, comparisons among studies are not straightforward due to the heterogeneity of the approaches used by different authors. In addition, many comparisons have so far chosen unrelated species, a method that is potentially confounded by phylogenetic differences. The most direct approach to identifying the determinants of invasiveness appears to be to make comparisons between related invasive and non-invasive, or less invasive, introduced species.

Knowledge of the invaders’ reproductive biology is important when attempting to manage them since it determines the continued survival of invasive populations even after eradication of the mother plants. I compared germination rates in different environmental conditions among three Impatiens: the invasive I. glandulifera; the naturalized I. parviflora; the casual I. balfourii. Greater seed mass and seed production rates, lower mortality, earlier germination, a shorter stratification period and the capacity to germinate under a wider range of conditions are factors that may contribute to the greater invasiveness of I. glandulifera. Similarly, lower seed mass and seed production rate, and higher mortality could contribute to explaining the lack of success of I. balfourii as an invader. The lack of success of I. parviflora (which has a very limited distribution in Ireland but which is invasive in central and eastern Europe) may be due, in terms of germination, to its requiring a longer and colder stratification period in order to break dormancy.

This thesis further explored the ecophysiology of these three introduced species. In particular, I assessed and compared traits that have been frequently suggested as contributing to plants’ invasiveness. From the characterisation of traits in a common high-resource environment, it emerged that I. glandulifera and I. balfourii have similar ecological needs. They also showed similar leaf morphological and photosynthetic traits. I then assessed and compared these two species in terms of traits related to growth, biomass allocation, leaf morphology, photosynthesis and efficiency, and in terms of plasticity: in one case, growing seedlings under two different light intensities; in another, growing plants under three different water regimes. Across the set of experiments, the invasive I. glandulifera showed consistently better performances than I. balfourii for the growth-related traits. I observed less consistency in terms of the leaf morphological, photosynthetic and use-efficiency traits, and in terms of plasticity. These characteristics were generally not significantly different between species, or were even found to be inferior in the invader.

Finally, I assessed the potential for hybridization between I. glandulifera and I. balfourii. The need for investigation into the potential for this cross is due to the fact that hybridization is suggested as a factor that may enhance invasiveness. The possibility of natural pollen transfer between the two species exists since they co-occur, the flowering time overlaps, and the pollinators switch from one species to the other. Moreover, hybridizations within the genus Impatiens are reported to occur between wild populations. I found that the heterospecific crosses produced seeds. However, even if the possibility of hybridization is excluded (due to the lack of germination of the hybrids), the fact that the heterospecific cross produced seeds represents the possible presence of incomplete reproductive barriers.

The results of this thesis highlight traits, such as reproductive capacity and growth characteristics, which are possibly involved in determining the greater invasive capacity of I. glandulifera. However, the possibility of I. balfourii also becoming invasive in Ireland cannot be excluded. This species showed a very high germination rate in some of the experimental conditions, suggesting that, in the right environments, it has great reproductive potential. In addition, on the basis of its ecophysiological characteristics, it could become invasive, in particular in disturbed high-light environments. Under climate warming, I. balfourii could possibly experience the favourable conditions required to invade cooler regions such as Ireland as well as more northern regions of Europe.